1. Field of the Invention
The invention generally relates to supramolecular complexes designed to convert light energy into chemical energy. Specifically, these supramolecular complexes produce hydrogen from water. The conversion of light energy into chemical energy could occur using other chemical feedstocks, for example, the reduction of carbon dioxide. In particular, the invention provides supramolecular complexes that absorb visible light and undergo charge transfer, leading to the collection of electrons at a reactive metal center, and the reduction of water to hydrogen.
2. Background of the Invention
The multi-electron reduction of substrates is a key area of research, especially as it relates to energy production, light to energy conversion, and water splitting. The splitting of water is an energetically uphill process, but one that can thermodynamically be accomplished through the use of solar energy. The relevant equations for water splitting are:4H2O+4e−→2H2+4OH−4OH−→O2+4e−+2H2OTherefore, for even the production of only one hydrogen molecule, a multi-electron process is necessary. One of the major impediments to the efficient conversion of solar energy is a need for an understanding of multi-electron catalysis.
The area of supramolecular chemistry seems an ideal forum in which to study multi-electron chemistry1, especially using covalently linked ruthenium charge transfer chromophores2. Nocera3 recognized the importance of these multi-electron processes and recently made significant progress in this area using multi-metallic complexes. The design of a functioning device for photoinitiated electron collection within a single molecular unit has been described.4 It was shown that {[(bpy)2Ru(dpb)]2IrCl2}(PF6)5 (bpy=2,2′-bipyridine and dpb=2,3-bis(2-pyridyl)benzoquinoxaline) was able, through a series of steps, and two single photon excitations, to become doubly reduced, storing the electrons on the π* orbitals of the dpb bridging ligand. Only one other report has shown photoinitiated electron collection in a molecular system that remains intact following electron collection.5 The systems reported by MacDonnell and Campagna use bridging ligands with an extended structure in which the spectroscopic orbital for the metal to ligand charge transfer (MLCT) transition is different than the lowest lying acceptor orbital, allowing for the functioning of these promising systems. One common feature of these successful systems is that the orbital accepting the first optically populated electron is different from the orbital that is involved in the second optical excitation. This feature of these functioning systems is maintained in all the proposed Rh centered systems. Bocarsly reported an interesting approach to photoinitiated electron collection in which the fragmentation of the supramolecular assembly following electron collection serves as a driving force for this reaction.6,7 Bocarsly has exploited an unstable PtIII redox state in [(CN)5FeII(CN)PtIV(NH3)4(NC)FeII(CN)5]4− and related systems to lead to electron collection at a metal leading to complex fragmentation. Bocarsly showed excitation of the metal to metal charge transfer (MMCT) state directly leads to a net two electron charge transfer that dissociates the complex into two FeIII and one PtII species.
Few molecular systems exist that successfully use light energy to collect electrons. There is thus an ongoing need for the discovery of molecular systems capable of electron collection, particularly when the systems utilize the electrons to carry out useful catalytic reactions such as splitting of H2O to produce H2, and particularly when the molecular systems remain intact following electron collection.